The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer structures is well known in the art. Such technology is of increasing interest in the fabrication of very dense multilayer integrated circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Significantly improved substrates for the fabrication of such circuits are disclosed and claimed in Hang et al., U.S. Pat. No. 4,256,796, issued Mar. 17, 1981, the disclosure of which is incorporated herein by reference. The Hang et al. substrates are metal coated with an improved porcelain composition comprised of a mixture, based on its oxide content, of magnesium oxide (MgO) or mixtures of magnesium oxide and certain other oxides, barium oxide (BaO), boron trioxide (B.sub.2 O.sub.3) and silicon dioxide (SiO.sub.2).
While the porcelain-coated metal substrates of Hang et al. represent a significant improvement over previously known substrate materials, they are disadvantageous only in being incompatible or poorly compatible with commercially available thick-film inks. This invention is concerned with improved copper conductor inks compatible with the Hang et al. substrates.
It is generally recognized that it is advantageous to include bismuth oxide in copper conductor inks to improve the solderability of films formed therefrom. Conventionally, such inks contain from about 5 percent up to as much as 10 percent by weight bismuth oxide. We have found that bismuth oxide in amounts in excess of about 3 percent by weight of the ink formulation will frequently cause a reaction at the point of contact between a conductor film and a resistor. This results in a significantly increased resistance at the point of contact, a distinct disadvantage.
In accordance with this invention, copper conductor inks are provided which are compatible with the Hang et al. substrates, and are characterized by good solderability without the aforementioned point of contact reaction with resistors.